The long-term goal of our research is to understand the molecular and cellular mechanisms governing morphogenesis of the neural tube in vertebrate embryos. The proposed experiments are significant because failure of neural tube morphogenesis is the second most common human birth defect. Our focus is on a cell shape change called apical constriction, a conversion of columnar cells into wedge-shaped cells, that contributes importantly to neural tube closure. Apical constriction involves accumulation of actin filaments at the apical surface of polarized epithelial cells. The molecular mechanisms underlying this process are unknown, but we have shown that a single protein, Shroom, is sufficient to induce both apical actin assembly and apical constriction. Shroom is essential for neural tube closure. This proposal aims to understand how Shroom-mediated reorganization of the actin cytoskeleton influences cell shape changes that are necessary for neural tube closure. We propose to determine the molecular mechanisms of Shroom-mediated actin assembly. We hypothesize that the actin regulators, Arp2/3, formin, and Mena are required for this process. We will test this using a gain-of-function assay for Shroom activity and inhibitors of the actin regulatory proteins. We suggest that cell behaviors associated with apical constriction are inter-related and controlled by Shroom. We will address this with loss-of-function experiments in Xenopus embryos and confocal imaging. Finally, we suggest that a Shroom-related gene, called APXL, controls apical constrictions in cells that lack Shroom expression. We will use both gain- and loss-of-function strategies to test this hypothesis. This work will help to elucidate the regulatory networks that govern cell shape-change in general and neural tube closure in particular. Moreover, this work has the potential to shed light on the underpinnings of human neural tube defects.